1. Field of the Invention
The present invention relates to a refractive index variable element that permits greatly varying the refractive index thereof by utilizing electrons and light and to a method of varying refractive index.
2. Description of the Related Art
In an optical or electronic function element or system which uses light as an information medium, it is absolutely necessary to control the refractive index of a component material or element. This is because the propagation characteristics of light are governed by the refractive index. Therefore, it is important to design an element so as to establish prescribed refractive index distribution, to arrange a material with a prescribed refractive index in the element, or to vary the refractive index of the element, not only in an optical waveguide and an optical fiber but also in an optical switching device and an optical recording device.
The known methods for greatly varying the refractive index utilize, for example, (1) Stark shift, (2) Franz-Keldysh effect, (3) Pockels effect, (4) Kerr effect, (5) orientation variation, (6) level splitting by magnetic field, (7) Cotton-Mouton effect, (8) optical Stark shift, (9) absorption saturation, (10) electromagnetically induced transparency (EIT), (11) photoisomerization, (12) structural change by light irradiation, (13) photoionization, (14) piezoreflection effect, (15) thermal band shift, (16) thermal isomerization, and (17) thermally-induced structural change. Techniques of varying the refractive index with the Pockels effect are disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2002-217488, Japanese Patent Disclosure No. 11-223701, and Japanese Patent Disclosure No. 5-289123.
The refractive index can be represented by a complex number in which a real part thereof denotes the refractive index in the narrow sense and an imaginary part thereof denotes absorption. In the mechanisms for the refractive index variation cited above, the variation in the real part of the complex refractive index is large in the absorption region and the absorption edge, but is small, i.e., not larger than 1%, in the non-absorption region. Also, an optical function device utilizing variation in absorbance, such as a light-absorption type optical switch, is being studied. However, the absorption implies that the intensity of the light beam carrying the information is lowered. Such being the situation, it is desirable that the real part of the complex refractive index can be greatly varied in a non-absorption wavelength region. Among the refractive index variable materials, the liquid crystal exhibits an exceptionally large variation not smaller than 10% in the real part of the complex refractive index in the non-absorption wavelength region. This is because the variation in the refractive index in the liquid crystal is brought about by the variation in orientation, not by the variation in the electronic polarizability. Taking into consideration of application of a refractive index variable material to an optical function device, however, a liquid refractive index variable material such as liquid crystal can be applicable to significantly limited fields.